1. Field of Invention
The present invention relates generally to the field of high-speed digital signals and services provided to subscribers via installed twisted-pair circuits. More specifically, the present invention is related to a system and method of using optical fiber technology to extend the range over which such signals and services may be provided.
2. Discussion of Prior Art
The most ubiquitous medium for transport of electronic communications is the Public Switched Telephone Network (PSTN), which was designed to provide two-way analog voice communications. This same network is capable of being used for digital communications by employing traditional xe2x80x9cmodems,xe2x80x9d but fundamental physical constraints severely limit the speed reliably attained by such devices. These restraints include the bandwidth limitations (under 4 KHz) of the PSTN and the signal-to-noise ratio (about 30 dB) of the medium carrying the signals.
The signal-to-noise ratio is derived from physical properties of subscriber lines utilized to transport the analog signals between the subscriber""s premises and the Central Office (CO), namely Unshielded Twisted Pairs (UTP) of copper wire. The attenuation of the desired signals in these wires, and the pickup of various undesired noises by these wires, result in limited signal-to-noise-ratio, and ultimately to restriction of the length of UTP which can be deployed.
The bandwidth limitation is derived from the fact that the PSTN was designed for voice transmission. Early telephones could only provide 4 KHz of bandwidth, a bandwidth sufficient for basic speech communications. Enforcing low bandwidth increases the efficiency of both Frequency Domain Multiplexing (FDM) analog systems and Time Domain Multiplexing (TDM) digital systems which carry multiple conversations on a single cable.
The 4 KHz bandwidth constraint is imposed by the network and is not a limitation of the subscriber lines themselves. Direct connection to the UTP affords much higher bandwidth and hence support for much higher rate digital communications. Recently, technologies termed collectively Digital Subscriber Line (DSL) have been employed to exploit these higher rates. The different variants of DSL are collectively referred to as xDSL (e.g. HDSL, ADSL and VDSL).
A typical xDSL system is illustrated in FIG. 1. At the CO there is a DSL access multiplexer (DSLAM) 100, which contains a bank of xDSL Terminal Units (xTU) and a mechanism for combining all the digital information to and from these xTUs into a single information stream in order to interface with a high-speed network, such as the Internet. At the other end of the subscriber lines 102a-102e are individual remote Terminal Units 104a-104e located at the subscriber""s premises. In order to differentiate between xTUs based on location, CO-based xTUs are typically designated xTU-C, while remote xTUs are designated xTU-R. Each xTU-R communicates with a corresponding xTU-C of DSLAM 100, extracting the digital information contained in the xDSL signal and forwarding it to the premises distribution network.
As aforementioned, xDSL technologies provide high-speed digital communications by exploiting frequencies within the physical bandwidth of the subscriber lines 102a-102e, but above the 4 KHz utilized by the Plain Old Telephone Service (POTS). For example, standard ADSL uses frequencies between about 30 KHz to about 1104 KHz; the lower portion of this spectrum (up to 138 KHz) being for upstream transmission from the subscriber, and the upper portion for downstream transmission to the subscriber.
Due to the attenuation of signals in UTP lines becoming stronger with increasing frequency, xDSL technologies which utilize very high frequencies are subject to extremely high attenuation factors. In addition, unwanted pickup of radiation from adjacent lines, a phenomenon known as cross-talk, also becomes more pronounced with increasing frequency. Because of these effects the signal-to-noise-ratio declines rapidly with increasing frequency. Conversely, as the distance from the subscriber to the CO increases, achievable data rates diminish. The maximum distance obtainable by the xDSL technology at a given data rate is called its maximum reach. The maximum reach of ADSL at 1.5 Mbps downstream is typically about 18 Kft (5.5 Km); for ADSL at 8 Mbps downstream this is reduced to about 11 Kft (3.3 Km); while VDSL at 52 Mbps downstream has a maximum reach of only 1 Kft (300 m).
xDSL service providers wish to provide the highest data rates to as many customers as possible. Unfortunately, because of the physical properties of UTP subscriber lines just described, these two aims are mutually incompatible. Higher data rates can only be achieved for shorter distances, thus restricting the number of reachable subscribers. Present xDSL technologies can supply the highest rates (e.g. VDSL) only to subscribers serviced by short lengths of UTP, lower rates (ADSL, HDSL) to more distant customers, and must declare distant customers ineligible for any type of xDSL access.
One solution proposed to this problem is so-called Fiber To The Cabinet (FTTC), scenario depicted in FIG. 2. With FTTC, digital data is transferred over fiber optic cable 202 from the CO 200 to a street cabinet containing the DSLAM 204. Subscriber lines 206a-206e from the cabinet to the xTU-Rs 208a-208e at the subscribers"" premises are kept to minimal lengths, thus enabling the highest data rates to be obtained. This scenario, and similar ones such as Fiber To The Basement (FTTB), are commonly used for VDSL service.
However, FTTC is not a fully satisfactory solution because of the inhospitality of the street cabinet environment. These cabinets obviously place restrictions on the physical size of equipment, and due to limited power and inadequate heat dissipation they severely restrict power consumption. These constraints have impeded successful mass deployment of FTTC-DSL solutions since high-speed DSL modems require sophisticated signal processing and high power digital and analog circuitry.
ADSL was originally designed for video on demand services. In this context U.S. Pat. No. 5,534,912 to Kostreski et al. suggests utilizing a fiber optic cable to extend ADSL reach. A plurality of video channels is arranged at a Central Office into ADSL format and, together with a provisioning channel, multiplexed into a composite spectrum. This composite spectrum is then transmitted to an intermediate distribution point, remote from the Central Office, over analog optical fiber. The composite spectrum is split and applied individually to channel selection mixers associated with the subscribers serviced by the intermediate distribution point. However, Kostreski et al. does not teach the use of the optical fiber to extend the range that general xDSL services (i.e., the transmission of arbitrary data) can be provided to allow additional subscribers to be reached while providing the high data rates available to users proximate to the CO to these additional subscribers. Rather, Kostreski et al., teaches the streaming of multiple video feeds via the channels to a subscriber and the provision of a video library for streaming video on demand to a subscriber utilizing one of the channels. Kostreski et al. also does not teach the connection of the DSLAM at the CO to a high-speed general-purpose network, such as the Internet, to provide subscribers the capability of general-purpose communications between their subscriber premises equipment, such as personal computers, with equipment connected to the high-speed general-purpose network.
Whatever the precise merits, features and advantages of the above cited reference, it does not achieve nor fulfill the purposes of the present invention.
A system and method for extending the range over which high-speed digital data from arbitrary communications networks can be transmitted using pairs of copper wire such as lines serving subscribers of the conventional telephone system. The digital data is assumed to be converted in the customary manner into an analog signal suitable for transmission over the line. In the present invention a broadband analog link capable of transferring signals over long distances is inserted at some point along the length of the line. In order to extend the range for multiple subscribers the analog signals destined for and emanating from the different subscribers are multiplexed in the frequency domain into a single composite analog signal for transmission over the broadband analog link.
In the preferred embodiment, the present invention utilizes an analog fiber optic link to extend the distance over which xDSL services can be provided between a Central Office (CO) and multiple subscribers"" premises. We assume that a DSL access multiplexer (DSLAM), located at the Central Office, receives data from and sends data to an arbitrary digital communications network. In the downstream direction the DSLAM generates a plurality of analog signals, each of which contains the data required to be transmitted to a single DSL subscriber. A centrally located analog fiber optic transceiver connected to the DSLAM receives the plurality of signals from the DSLAM and constructs a frequency division multiplexed (FDM) signal from the plurality of signals. This FDM signal is transmitted to a remotely located fiber optic transceiver via an analog fiber optic link. At the remotely located fiber optic transceiver, the plurality of DSL signals is reconstructed from the FDM signal and each individual signal is transmitted to its corresponding DSL subscriber via the appropriate subscriber line. Similarly, in the upstream direction, the plurality of analog signals emanating from multiple DSL subscriber modems is converted into a composite FDM signal and transmitted by the remotely located fiber optic transceiver over the fiber optic link to the centrally located fiber optic transceiver. There the DSL signals are reconstructed and presented to DSLAM which extracts the digital data from each, multiplexes these data into a composite digital data stream, and forwards this digital data to the arbitrary digital communications network.
In an alternative embodiment, the present invention additionally provides Plain Old Telephone Service (POTS) to a subscriber. In one implementation of this embodiment, a POTS splitter is provided between the subscriber xDSL modem and the remotely located fiber optic transceiver. The POTS splitter separates the POTS service signal from the xDSL service signal for each subscriber line. The xDSL service signals are then transmitted via the fiber optic link to the CO as described in the preferred embodiment, while the POTS service signals are transmitted to the CO via the legacy subscriber line. In a second implementation of this embodiment, the POTS audio and signaling signals are converted to an appropriate format and sent via the fiber optic link. The supply voltage required at the subscriber premises is injected between the remotely located fiber optic transceiver and the subscriber line by appropriate circuitry.
In another embodiment of the present invention, the bandwidth of the channels in the FDM signal are dynamically set depending upon the bandwidth requirements of each particular subscriber. The configuration for the bandwidth of each channel may be set at the CO and communicated to the remotely located transceiver via a management protocol.